Polyamide shock absorber piston from terephthalic acid and branched diamine

ABSTRACT

A shock absorber piston especially suitable for use in automobiles and like vehicles wherein the piston is in contact with a damping fluid and is subject to repeated stresses under varying degrees of pressure and temperature; the piston is made of an amorphous polyamide characterized by a viscosity of about 100 to 200 in a 0.5 percent by weight solution in m-cresol at 25*C.

United States Patent 11 1 Schneider et al.

[ Nov. 5, 1974 1 POLYAMIDE SHOCK ABSORBER PISTON FROM TEREPI-ITIIALIC ACID AND BRANCIIED DIAMINE [75] Inventors: Johannes Schneider; Wolfgang Pungs, both of Troisdorf, Germany [73] Assignee: Dynamit Nobel Aktiengesellschaft,

Troisdorf, Germany [22] Filed: July 6, 1971 [21] Appl. No.: 160,120

[30] Foreign Application Priority Data 3,150,117 9/1964 Gabler 260/78 R 3,198,771 8/1965 Gabler 260/78 R 3,294,758 12/1966 Gabler.... 260/78 R 3,393,210 7/1968 Speck 260/78 R 3,416,302 12/1968 Knospe 260/78 R 3,449,299 9/1969 Schreider et al 260/78 R 3,454,536 7/1969 Schade et al 260/78 R 3,728,312 4/1973 Schneider et a1. 260/78 R 3,728,312 4/1973 Schneider 260/78 R OTHER PUBLICATIONS Chemical Abstsracts, Vol. 58, 6459g, (1963).

Polyamide Resin Floyd, Second Edition, (1966), p. 157.

Primary Examinerl-larold D. Anderson Attorney, Agent, or Firm-Craig & Antonelli [57] ABSTRACT A shock absorber piston especially suitable for use in automobiles and like vehicles wherein the piston is in contact with a damping fluid and is subject to repeated stresses under varying degrees of pressure and temperature; the piston is made of an amorphous polyamide characterized by a viscosity of about 100 to 200 in a 0.5 percent by weight solution in m-cresol at 25C.

4 Claims, No Drawings POLYAMIDE SHOCK ABSORBER PISTON FROM TEREPHTHALIC ACID AND BRANCHED DIAMINE This invention relates to shock absorber pistons which are guided displaceably in a cylinder and are attached to a piston rod in a damping fluid, e.g., a hy-f draulic liquid, so that the cylinder is subdivided into! two operating or working chambers of varying volume,l and which are produced from a thermoplastic materiall having a polyamide base. Shock absorber pistons arel utilized as a damper element in industry, predominantly in the automobile industry. Heretofore, these pistons have been manufactured mainly of metalsi However, this conventional material is, in most cases, far from satisfactory for meeting the heterogeneous requirements found in practice.

In addition to the chemical stability and the indifference or inertness of the shock absorber material with respect to the damping fluid, the following properties are desirable: (1) easy handling due to low specific gravity of the material, (2) economical manufacture, (3) corrosion resistance, (4), a dense, smooth surface on the material, (5) favorable sliding properties, (6) high pressure resistance and rigidity, and (7) good dimensional stability at varying temperatures.

Many of these properties are provided in synthetic resins. Therefore, there have been many attempts to introduce shock absorber pistons made of various polymers to industry. However, a satisfactory solution,

heretofore, has been impossible for the reason that no synthetic resin or polymer was found which fulfills all above-mentioned requirements in their entirety and which can be handled easily during the processing thereof so that even complicated shock absorber pistons with ribs and perforations can be readily manufactured without any secondary treatment, e.g., refinishmg.

drocarbons and chlorosulfonated polyethylene do exhibit a high chemical resistance, but are too soft and are unsuitable for use in making such pistons. Shock absorbers made of thermosets, i.e., thermosetting resins, fulfill the requirements of thermal dimensional stability according to Martens, but are extremely sensitive to breakage and impacts and thus do not meet the requirements in practical operation.

Polyolefins, such as polyethylenes or cross-linked polyethylenes, polypropylenes and poly-4- methylpentene satisfy a number of requirements, but exhibit the disadvantage of a pronounced tendency to flow under pressure and temperature stresses. In case of pistons having larger dimensions, the required structural rigidity is likewise absent in polyolefins, due to the lower modulus of elasticity. Besides, shaped articles of polyolefins tend to warp under temperature stress, due to the high coefficients of thermal expansion.

With the use of partially crystallizing polyamides, brittleness phenomena are observed at the temperatures of about 100C. which often occur in practice. Also, the low dimensional stability under temperature stress is found to be a disadvantage.

Polyvinyl chloride exhibits a good chemical resistance, but cannot be employed at temperatures of above 80C. due to its low dimensional stability at high temperatures. Polyacetals have a high pressure resistance and good abrasion resistance, but tend to exhibit undesired warping phenomena due to shrinkage with age.

10 isfies all the conditions to be fulfilled in practice partic- Fluorinated or fluorochlorinated high-molecular hyularly in the automotive industry.

Thus, this invention contemplates shock absorber pistons which are characterized in that the pistons are produced from amorphous polyamides which contain residues of aliphatic, straight-chain or branched diamines or aromatic diamines and residues of aliphatic or aromatic dicarboxylic acids, and which exhibit a viscosity of about 100 to about 200, preferably about 124 to about 150.

Examples for suitable diamine components for the manufacture of the polyamides of this invention are hexamethylenediamine; nona-, deca-, or dodecamethylenediamine; and furthermore alkyl-substituted diamines, such as 2-methyl-4-ethylhexamethylenediamine; 2 ,2,5,5-tetramethylhexamethylenediamine; 3- isopropylhexamethylenediamine; 3-isooctylhexamethylenediamine; 3-isododecylhexamethylenediamine; 2,4-diethyloctamethylenediamine; 2,2,4-trimethylhexamethylenediamine; 2,4,4-trimethylhexamethylenediamine; or also cyclic diamines, such as Z-aminomethyl- 3,5,5-trimethylcyclohexylamine; or furthermore aromatic diamines, such as phenylenediamine and xylylenediamine. Additionally suitable are diamines of the general formula:

wherein R represents hydrogen or an alkyl group of 1-4 carbon atoms, R represents an unsubstituted or alkylsubstituted alkylene group of l-l0 carbon atoms in the chain, or an unsubstituted or alkyl-substituted phenylene residue (the alkyl groups having 14 carbon atoms), and R represents an alkyl group of l-4 carbon atoms. Exemplary diamines represented by this formula are, for example, 1,3-diaminobutane; 2,5- diamino-3 ,4-dimethylhexane; 1,5-diamino-4- isopropylhexane; 2,7-diamino-4-methyloctane; 3,5- diaminoheptane; l,7-diarnino-4,4-dimethyloctane; 1,7- diamino-4,4,6-trimethyloctane; l,7-diamino-4,4 ,8- trimethylnonane; or a, oz'-diamino-l ,3-diethylbenzene. Suitable dicarboxylic acid components for the production of the polyamides can be, for example, aliphatic dicarboxylic acids, such as adipic or sebacic acid, particularly in conjunction with hexamethylenediamine and p,p-diaminocyclohexylpropane or -methane, and aromatic dicarboxylic acids, such as terephthalic acid, isophthalic acid, or naphthalene dicarboxylic acid.

The polyamides are to exhibit a viscosity of about 100 to about 200, preferably about 124 to about 150, wherein the measurement of the viscosity number is conducted in accordance with the German Industry Standard testing procedure designated as DIN 53727 in a 0.5 percent by weight solution in m-cresol at 25C..

These polyamides can be prepared basically in accordance with all processes customary for the production of conventional polyamides containing dicarboxylic acid and diamine residues. Thus, it is possible, for example, to polycondense the aqueous, concentrated solution of the salt of a dicarboxylic acid and a diamine in the melt, first under pressure and then under expansion at an elevated temperature. In this connection, it is also possible to circumvent the pressure stage by subjecting the salt to a preliminary condensation in highboiling solvents, and apply a vacuum in the last stage of the polycondensation.

It is also feasible to react lower alkyl esters of the dicarboxylic acids with practically equimolar amounts of diamine in the presence of water and to polycondense the product like an aqueous salt solution. Instead of starting with lower alkyl esters, the starting compounds can also be diaryl esters of the dicarboxylic acids, and in this case the concomitant use of water can be eliminated. As for the above-mentioned polyamides and/or the preparation thereof, note, for example, US. Pat.

pistons according to the present invention exhibit, upon storage for a longer period of time, practically no dimensional changes. No deformation or destruction under practical conditions was observed. Just as favorable is the minor and constant process shrinkage, so that the manufacture of complicated and finely structured shock absorber pistons is very easily possible.

The transparent shock absorber pistons of this invention exhibit a smooth and non-porous surface and can be manufactured on commercial-type injection molding machines. The transparent polyamides used for the manufacturing process can, of course, be mixed with coloring agents, e.g., pigments, so that an opaque or transparent coloring of the shock absorber pistons is made possible. 7

In the following table, several properties are listed of a number of polyamides on the basis of aromatic dicarboxylic acids usable according to this invention for the manufacture of shock absorber pistons, and measured No. 3,150,117; British Pat. No. 1,049,987; and Belgain in accordance with German Industrial Standards P t. No. 723,154.

(DIN).

TABLE 1 Test Procedure According to DIN Terephthalic Acid Polvamides From Dimethyl Terephthalate Dimethyl Terephl,7-Diamino4,4- 2,2,4-/2,4,4-Trimethylthalate 70% 2,2,4-/ dimethyloctane hexamethylenediamine 2,4,4-Trimethylhexameth lenediamine examethylenediamine Viscosity S3 727 I24 140 150 Density g./cm 53 479 1.09 1.12 l.l4

Critical modulus of rupture kp/cm 53 452 1090 1250 1140 upon flexing Tensile strength kp/cm 53 455 830 850 815 Impact strength kp cm/cm 53 453 rlg n.g. n.g. Ball indentation hardness kp/cm 53 456 1400 1400 1400 Linear coefficient of l/degree C. 61 10 60 10 60 10 thermal expansion Heat distortion stability degrees C. 53 458 I08 100 10] according to Martens Process shrinkage percent 0.6 0.5 0.5

Viscosity in a 0.5% by weight solution in m-cresol at 25C.

The aforementioned polyamides can be processed to transparent shaped articles in an injection molding, extrusion, and press molding process. In order to avoid The properties of polyamides of adipic acid or sebacic acid, hexamethylenediamine or p,p'-diaminocyclohexylpropane or p,p-diaminocyclohexylmethane,

undesired effects during processing, such as, for examandv Caprolactam are approximately on the following ple, the formation of streaks in and/or on the shaped Order Of agnitude! article, it proved to be particularly advantageous to I condition the abovementioned polyamides in the form TABLE 2 of chips or granules for about 14 hours at a temperature in the range of l lO-150C., preferably 140C., in viscosit Test 22" an inert gas atmosphere, e.g., nitrogen, carbon dioxide, y

argon, helium, or the like, under a vacuum, e.g., from Pensity gilcm3 53 479 700 to 10 mm Hg. Critical modulus f kp/cm 53 452 1300 Shock absorber pistons can be readily manufactured ifg gf kp/Cm, 53 455 800 from the above-dlsclosed amorphous polyamides, after carrying out the conditioning step, by means, of the in- Impact Strength P 'Cm/cmz 53 453 sjection molding process. They excel in their high resis- Ba" indemaion kp/cmz 53 446 1400 tance against aliphatic and aromatic hydrocarbons, hardness against mineral oils, as well as against acids and alkalis. Linear), coefficient of l/degree 60 10-6 The mechanical strength and the heat dlstortlon stabllthermal expansion accordmg to M r (DIN are very good Heat distortion stability degrees c. 53 458 95400 The thermal coefflclent of expanslon ls small, and the according to Martens abrasion resistance and the pressure resistance correpercent 0 4 spond to practical requirements. The shock absorber Process shrinkage Examples for the production of shock absorber pis- EXAMPLE 1 On a commercial screw-type injection molding machine, consisting of an injection unit, a nozzle, and a mold, chips of amorphous polyamide from the dimethyl ester of terephthalic acid and a mixture of 2,2,4- and 2,4,4-trimethylhexamethylenediamine are processed after having been conditioned beforehand, i.e., at 140C. in a nitrogen atmosphere. The polyamide exhibited a viscosity number of 140 and was processed in the screw cylinder at temperatures of 240-320C., preferably 260-305C., into transparent shock absorber pistons. The injection molding pressure was about 200 kp/cm the mold temperature was 90C. The total cycle of the process was 40 seconds. Since the shock absorber pistons exhibit a high strength immediately upon removal from the mold, they are directly suitable for the subsequent assembly operation.

EXAMPLE 2 EXAMPLE 3 The shock absorber pistons manufactured by the procedures outlined in Examples 1 and 2 and pistons produced by other suitable polyamides having a viscosity of 100-200 were evaluated by DIN testing procedures and found to have a specific gravity in the range of from 1.09 to 1.14 g./cm a tensile strength from 750 to 850 kp/cm an impact strength of no failure according to DIN 53453; a hardness of about 1400 kplcm a linear coefficient of thermal expansion of about 6010 1C. a heat distortion stability according to Martens of from to 120C.; a process shrinkage of from 0.4 to 0.6 percent; and a critical modulus of rupture upon flexing of 1000 to 1250 kp/cm This combination of properties and the corrosion resistance of these polyamides has been found to be especially suitable for producing shock absorber pistons which satisfy the demand of the automotive industry.

What we claim is:

l. A shock absorber piston, adapted to be guided in a cylinder containing a damping fluid and to be attached to a piston rod, said piston being produced from amorphous polyamide consisting essentially of the polymeric condensation product of the dimethyl ester of terephthalic acid and a mixture of 2,2,4- and 2,4,4- trimethylhexamethylenediamine, said polyamide exhibiting a viscosity of about to about 200 measured in a 0.5 percent by weight solution in m-cresol at 25C. and prior to the formation of said piston having been conditioned for 8 to 16 hours at a temperature from about C. to about C. in an inert gas atmosphere under vacuum.

2. A shock absorber piston, adapted to be guided in a cylinder containing a damping fluid and to be attached to a piston rod, said piston being produced from amorphous polyamide consisting essentially of the polymeric condensation product of residues of an aliphatic branched diamine or mixtures thereof and residues of an aromatic dicarboxylic acid, said polyamide exhibiting a viscosity of about 100 to 200 measured in a 0.5 percent by weight solution in m-cresol at 25C. and prior to formation of said piston having been conditioned for 8 to 16 hours at a temperature of about 110C. in an inert gas atmosphere under vacuum.

3. The shock absorber piston of claim 2 in which the amorphous polyamide is produced, with respect to the acid residues, from terephthalic acid; and, with respect to the diamine residues, from a diamine selected from consisting of 2,2,4-trimethylhexame- 2,4,4-trimethylhexamethylenediathe group thylenediamine; mine; and l,7-diamino-4,4-dimethyloctane.

4. The shock absorber piston of claim 2 in which the amorphous polyamide is produced from the dimethyl ester of terephthalic acid and from the mixture of diamines of 2,2,4- and 2,4,4-trimethylhexamethylenediamine, or up to about 30 percent of the diamine mixture replaced by hexamethylenediamine. 

1. A SHOCH ABSORBER PISTON, ADAPTED TO BE GUIDED IN A CYLINDER CONTAINING A DAMPING FLUID AND TO BE ATTACHED TO A PISTON ROD, SAID PISTON BEING PRODUCED FROM AMORPHOUS POLYAMIDE CONSITING ESSENTIALLY OF THE POLYMERIC CONDENSATION PRODUCT OF THE DIMETHYL ESTER OF TEREPHTHALIC ACID AND A MIXTURE OF 2,2,4- AND 2,4,4-TRIMETHYLHEXAMETHYLENEDIAMINE, SAID POLYAMIDE EXHIBITING A VISOCITY OF ABOUT 100 TO ABOUT 200 MEASURED IN A 0.5 PERCENT BY WEIGHT SOLUTION IN M-CRESOL AT 25*C. AND PRIOR TO THE FORMATION OF SAID PISTON HAVING BEEN CONDITIONED FOR 8 TO 16 HOURS AT A TEMPERATURE FROM ABOUT 110*C. TO ABOUT 150*C. IN AN INERT GAS ATMOSPHERE UNDER VACUUM.
 2. A shock absorber piston, adapted to be guided in a cylinder containing a damping fluid and to be attached to a piston rod, said piston being produced from amorphous polyamide consisting essentially of the polymeric condensation product of residues of an aliphatic branched diamine or mixtures thereof and residues of an aromatic dicarboxylic acid, said polyamide exhibiting a viscosity of about 100 to 200 measured in a 0.5 percent by weight solution in m-cresol at 25*C. and prior to formation of said piston having been conditioned for 8 to 16 hours at a temperature of about 110*C. in an inert gas atmosphere under vacuum.
 3. The shock absorber piston of claim 2 in which the amorphous polyamide is produced, with respect to the acid residues, from terephthalic acid; and, with respect to the diamine residues, from a diamine selected from the group consisting of 2,2,4-trimethylhexamethylenediamine; 2,4,4-trimethylhexamethylenediamine; and 1,7-diamino-4,4-dimethyloctane.
 4. The shock absorber piston of claim 2 in which the amorphous polyamide is produced from the dimethyl ester of terephthalic acid and from the mixture of diamines of 2,2,4- and 2,4,4-trimethylhexamethylenediamine, or up to about 30 percent of the diamine mixture replaced by hexamethylenediamine. 